The present application relates generally to the use of liquid desiccant membrane modules to dehumidify and cool an outside air stream entering a space. More specifically, the application relates to the use of micro-porous membranes to keep separate a liquid desiccant that is treating an outside air stream from direct contact with that air stream while in parallel using a conventional vapor compression system to treat a return air stream. The membrane allows for the use of turbulent air streams wherein the fluid streams (air, optional cooling fluids, and liquid desiccants) are made to flow so that high heat and moisture transfer rates between the fluids can occur. The application further relates to combining cost reduced conventional vapor compression technology with a more costly membrane liquid desiccant and thereby creating a new system at approximately equal cost but with much lower energy consumption.
Liquid desiccants have been used in parallel with conventional vapor compression HVAC (heating, ventilation, and air conditioning) equipment to help reduce humidity in spaces, particularly in spaces that either require large amounts of outdoor air or that have large humidity loads inside the building space itself. Humid climates, such as for example Miami, Fla. require a large amount of energy to properly treat (dehumidify and cool) the fresh air that is required for a space's occupant comfort. Conventional vapor compression systems have only a limited ability to dehumidify and tend to overcool the air, oftentimes requiring energy intensive reheat systems, which significantly increase the overall energy costs because reheat adds an additional heat-load to the cooling coil. Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from the air stream. However, liquid desiccant systems generally use concentrated salt solutions such as solutions of LiCl, LiBr or CaCl2 and water. Such brines are strongly corrosive, even in small quantities so numerous attempt have been made over the years to prevent desiccant carry-over to the air stream that is to be treated. One approach—generally categorized as closed desiccant systems—is commonly used in equipment dubbed absorption chillers, places the brine in a vacuum vessel which then contains the desiccant and since the air is not directly exposed to the desiccant; such systems do not have any risk of carry-over of desiccant particles to the supply air stream. Absorption chillers however tend to be expensive both in terms of first cost and maintenance costs. Open desiccant systems allow a direct contact between the air stream and the desiccant, generally by flowing the desiccant over a packed bed similar to those used in cooling towers and evaporators. Such packed bed systems suffer from other disadvantages besides still having a carry-over risk: the high resistance of the packed bed to the air stream results in larger fan power and pressure drops across the packed bed, thus requiring more energy. Furthermore, the dehumidification process is adiabatic, since the heat of condensation that is released during the absorption of water vapor into the desiccant has no place to go. As a result both the desiccant and the air stream are heated by the release of the heat of condensation. This results in a warm, dry air stream where a cool dry air stream was desired, necessitating the need for a post-dehumidification cooling coil. Warmer desiccant is also exponentially less effective at absorbing water vapor, which forces the system to supply much larger quantities of desiccant to the packed bed which in turn requires larger desiccant pump power, since the desiccant is doing double duty as a desiccant as well as a heat transfer fluid. But the larger desiccant flooding rate also results in an increased risk of desiccant carryover. Generally air flow rates need to be kept well below the turbulent region (at Reynolds numbers of less than ˜2,400) to prevent carryover. Applying a micro-porous membrane to the surface of these open liquid desiccant systems has several advantages. First it prevents any desiccant from escaping (carrying-over) to the air stream and becoming a source of corrosion in the building. And second, the membrane allows for the use of turbulent air flows enhancing heat and moisture transfer, which in turn results in a smaller system since it can be build more compactly. The micro-porous membrane retains the desiccant typically by being hydrophobic to the desiccant solution and breakthrough of desiccant can occur but only at pressures significantly higher than the operating pressure. The water vapor in an air stream that is flowing over the membrane diffuses through the membrane into the underlying desiccant resulting in a drier air stream. If the desiccant is at the same time cooler than the air stream, a cooling function will occur as well, resulting in a simultaneous cooling and dehumidification effect.
U.S. Patent Application Publication No. 2012/0132513, and PCT Application No. PCT/US11/037936 by Vandermeulen et al. disclose several embodiments for plate structures for membrane dehumidification of air streams. U.S. Patent Application Publication Nos. 2014-0150662, 2014-0150657, 2014-0150656, and 2014-0150657, PCT Application No. PCT/US13/045161, and U.S. Patent Application Nos. 61/658,205, 61/729,139, 61/731,227, 61/736,213, 61/758,035, 61/789,357, 61/906,219, and 61/951,887 by Vandermeulen et. al. disclose several manufacturing methods and details for manufacturing membrane desiccant plates. Each of these patent applications is hereby incorporated by reference herein in its entirety.
Conventional Roof Top Units (RTUs), which are a common means of providing cooling, heating, and ventilation to a space are inexpensive systems that are manufactured in high volumes. However, these RTUs are only able to handle small quantities of outside air, since they are generally not very good at dehumidifying the air stream and their efficiency drops significantly at higher outside air percentages. Generally RTUs provide between 5 and 20% outside air, and specialty units such as Make Up Air (MAUs) or Dedicated Outside Air Systems (DOAS) exist that specialize in providing 100% outside air and they can do so much more efficiently. However, the cost of a MAU or DOAS is often well over $2,000 per ton of cooling capacity compared to less than $1,000 per ton of a RTU. In many applications RTUs are the only equipment utilized simply because of their lower initial cost since the owner of the building and the entity paying for the electricity are often different. But the use of RTUs often results in poor energy performance, high humidity and buildings that feel much too cold. Upgrading a building with LED lighting for example can possibly lead to humidity problems and the cold feeling is increased because the internal heat load from incandescent lighting which helps heat a building, largely disappears when LEDs are installed.
Furthermore, RTUs generally do not humidify in winter operation mode. In winter the large amount of heating that is applied to the air stream results in very dry building conditions which can also be uncomfortable. In some buildings humidifiers are installed in ductwork or integrated to the RTU to provide humidity to the space. However, the evaporation of water in the air significantly cools that air requiring additional heat to be applied and thus increases energy costs.
There thus remains a need for a system that provides cost efficient, manufacturable and thermally efficient methods and systems to capture moisture from an air stream, while simultaneously cooling such an air stream in a summer operating mode, while also heating and humidifying an air stream in a winter operating mode and while also reducing the risk of contaminating such an air stream with desiccant particles.